1. Field of the Invention
This invention relates generally to optical oscillators, and more particularly to optical oscillators where the intracavity beam is maintained at a fixed position within an intracavity optical system to maintain wavelength stability of the output beam.
2. Description of the Related Art
Accordingly, what is needed is a system and method for providing a cost-effective-wavelength locker to stabilize the wavelength of an optical oscillator, such as a tunable laser or optical parametric oscillator. The wavelength locker produced should be reliable and stable over a range of temperatures.
One way of accomplishing wavelength stabilization is by use of feedback from a reference to control a wavelength-tuning element inside the optical oscillator. This tuning may be accomplished by temperature control of a gain medium, by adjustment of temperature or angular tilt or spacing of an intracavity etalon, by adjusting the angle of a prism, a grating, a mirror, or a birefringent filter, by adjusting the position of a coated tuning wedge to control the spacing of the equivalent etalon seen by the cavity, or by adjustment of the cavity length, or other suitable means. All of these approaches can be used in combination with feedback from an external reference spectrometer, an external reference interferometer or etalon, an atomic or molecular absorption line, or other suitable means. In this invention, the wavelength reference is already built into the optical oscillator, and stabilization of the wavelength is achieved by stabilization of the beam path relative to the internal reference. This internal reference includes wavelength dispersive optical elements and a slit, components that could also be used to create an external spectrometer.
Another way to accomplish wavelength stabilization is to injection-seed the oscillator with a beam from another oscillator that operates with a stable wavelength. This is commonly done in pulsed lasers to achieve narrow linewidth wavelength stabilized operation through injection seeding by a low power cw laser beam. The laser pulse is initiated by amplification of the narrow band light provided by the seed laser rather than being initiated by spontaneous emission within the gain medium. U.S. Pat. No. 4,955,027 (Wavelength Locked Laser Light Source) by Piper at el, describes a system in which laser output wavelength stability is enhanced through such an injection-seeding process.
In U.S. Pat. No. 5,809,048 (Wavelength Stabilized Light Source) by Shichijyo et al, a means of providing wavelength filtered optical feedback (using a birefringent Lyot filter) to a semiconductor laser is described as producing improved wavelength stability. This is an example of direct feedback from an external wavelength sensitive optical device to lock the wavelength of the oscillator, a type of injection seeding in which the injected signal is derived from the filtered output of the laser to be controlled.
In U.S. Pat. No. 4,583,228 (Frequency Stabilization of Lasers) by Brown et al, the wavelength stabilization of a semiconductor laser is based on a feedback signals derived from an external Fabry-Perot interferometer that were used to control both the drive current and the laser temperature. This is an example of electronic feedback that is derived from an external wavelength-sensing optical device. Electonic control is applied to critical laser parameters to control the wavelength.
In U.S. Pat. No. 6,393,037 (Wavelength Selector for Laser with Adjustable Angular Dispersion) by Basting et al, the wavelength and linewidth of a laser are controlled by use of signals generated in a linewidth and wavelength-monitoring unit, which samples the laser output beam. Control is provided through use of a signal processor that can direct prisms to rotate within the laser to change refraction angles to change laser wavelength and linewidth. This has some similarity to the invention described here in that both rely on stabilization or control by means of movement of an optical element.
In U.S. Pat. No. 5,017,806 (Broadly Tunable High Repetition Rate Femtosecond Optical Parametric Oscillator) by Edelstein et al, a synchronously pumped OPO (optical parametric oscillator) that can produce femtosecond light pulses is described. The output wavelength of the OPO is held stable by a feedback loop that controls the length of the OPO cavity. The feedback is derived from detectors that monitor the direction of shift of the output spectrum. Because the performance of such an OPO also depends on the wavelength stability of the mode-locked pump laser, it can clearly benefit from application of the present invention to stability of the pump laser wavelength.
In U.S. Pat. No. 4,932,030 (Frequency Stabilization of Long Wavelength Semiconductor Laser via Optogalvanic Effect) by Chung, wavelength stabilization is achieved by locking the output wavelength of a laser to a transition in an atomic absorber excited in an electrical discharge. In this case, the feedback loop used to control the laser was responding to an optogalvanic signal derived by means of dithering lock-in techniques.
The present invention differs from the techniques mentioned above in that the wavelength reference is located within the oscillator rather than externally, and the wavelength stability derives from feedback from an external position-sensing detector to maintain the intracavity beam position relative to the reference.
A convenient application of the present invention is in an ultra-short-pulse laser that already utilizes prisms to provide dispersion compensation that is necessary for production of the ultra short pulses, especially for pulses shorter than one picosecond in duration. In this case the prism sequence that is used for dispersion compensation can also be used as the wavelength reference needed for the stabilization of the output wavelength. Pulses as short as 100 femtoseconds in duration can only be produced if the laser spectrum has more than 6 nm of bandwidth (the full width at half maximum of the spectral spread of the laser output beam). In this case, the output wavelength can be defined as the wavelength halfway between the half-maximum wavelengths, and that is the wavelength that is kept stable against environmental changes by application of the invention.
Such a wavelength stabilized laser system would have applications to pumping of an optical parametric oscillator (angle-tuned, temperature-tuned, or wavelength-tuned), to creation of harmonics in non-linear crystals by processes that have phase-matching sensitivity (angle-tuned or temperature-tuned, for example), and to seeding of amplifier systems such as regenerative chirp-pulse amplifiers, for which stability of the amplified pulse is critically dependent on stability of the seed pulse wavelength. In addition, such lasers have applications to scanning microscopy systems for which enhanced wavelength stability is desirable for examination of samples that have wavelength sensitivity.